In 2015 we'll find out if Pluto's moon is hiding an ancient ocean

Artist's impression of Pluto and its moonsNASA, ESA and G. Bacon (STScI)

Icy, foreboding and named after the ferryman that delivers souls
across the river Styx, Pluto's moon Charon was always steeped in
plenty of drama and mystery. That was, until another four small moons were discovered orbiting the distant
planet, and it sort of ebbed into the mundane.

Now the celestial orb is due a renaissance, with the news from
Nasa that if its New Horizon spacecraft spots cracks on Charon's
1,043 km diameter surface when it flies over in July 2015 --
the first time we'll be able to get a high resolution snapshot of
the distant moon -- any cracks it identifies could be signs of an
ancient hidden ocean.

Surface temperatures on Charon are estimated to be about -229C,
far too cold to support a liquid water system -- which is rather
expected, considering it's 40 times farther from the Sun than Earth
is.
But researchers have already predicted that Charon's surface will
be littered with fractures, a telltale sign of ancient friction
going on beneath the surface which, as a consequence, would have
heated up the body's interior.

We've already seen these on the surface of Jupiter's Europa
satellite and on Saturn's Enceladus. And according to Alyssa Rhoden
of Nasa's Goddard Space Flight Centre, the data from New Horizons
will be able to clear up whether those fractures hide ancient
frozen oceans that once flowed beneath.

Cracks in Saturn's moon Enceladus taken by the Cassini spacecraft during its close flyby on 9 March and 14 July, 2005. NASA/JPL/Space Science Institute

Like Europa and Enceladu, Charon is thought to have had an
eccentric orbit in the early days of its evolution, which basically
means its orbit deviated from the traditional circular model. It
would have appeared to be oval, and this dynamic would have pushed
and pulled the tides in different ways, raising sea levels and
causing friction and therefore heat. This is partially because of
how Charon was formed. It's huge -- around half the size of Pluto
-- and is thought to have been created when Pluto bashed into
another planet-sized celestial body in the early days of the solar
system. At that time the gravitational pull between the two bodies
would have been extraordinary, pulling their respective surfaces
(and thus any water) towards each other. This would have made for a
"high-eccentricity phase" as the planet and moon gradually moved
away from one another, the moon speeding up and the planet slowing
down, with high tides caused by the massive amounts of underlying
friction.

Hence, it's predicted that the likes of Europa -- and now,
perhaps Charon -- may have had underground oceans for longer,
whilst this friction maintained the right temperature conditions
deep beneath the surface.

"Our model predicts different fracture patterns on the surface
of Charon depending on the thickness of its surface ice, the
structure of the moon's interior and how easily it deforms, and how
its orbit evolved," said Rhoden. "Depending on exactly how
Charon's orbit evolved, particularly if it went through a
high-eccentricity phase, there may have been enough heat from tidal
deformation to maintain liquid water beneath the surface of Charon
for some time. Using plausible interior structure models that
include an ocean, we found it wouldn't have taken much eccentricity
(less than 0.01) to generate surface fractures like we are seeing
on Europa."

If there are fractures on the surface, it would back up this
theory. But the stability of the planet and its moon today means an
underwater paradise is highly unlikely sadly. With no eccentric
orbit, and therefore no friction, Charon is destined to remain icy,
so far from the Sun. A celestial body of course also needs the
necessary atmosphere to support life, not just water, and it was
thought that Charon had no atmosphere. However, an article posted
in the journal Icarus speculates that Charon is in fact
sharing Pluto's atmosphere. That atmosphere is mostly of nitrogen,
though.

Either way, there's a lot of people excited for New Horizon's
fly-by in 2015.